Using fluidRow with a conditional statement

Question

The following app generates a dynamic UI based on the number of variables selected. A problem is that when the number of variables selected is odd, the app generates an extra UI that is not tied to any of the variables previously selected. I've tried include if statements within the fluidRow creation statement, essentially checking if there is a remainder and if so, I've tried to tell the app to insert a blank space, but this doesn't do the trick. Does anyone have any suggestions on how to fix the issue?

## libraries
library(tidyverse)
library(shiny)

ui <- fluidPage(
  selectInput(inputId = "var",
                 label = "vars:",
                 choices = colnames(mtcars),
                 multiple = TRUE),
  uiOutput("dynUI")
)

server <- function(input, output, session) {
  output$dynUI <- renderUI({
    row_idx <- length(input$var) %>% seq_len
    row_idx <- row_idx[row_idx %% 2 == 1]
    row_idx %>% 
      map(~fluidRow(column(width = 2, 
                           selectizeInput(inputId = paste0("var", .x), 
                                          label = paste(input$var[.x], "var:"),
                                          choices = c("this", "that"),
                                          multiple = FALSE)),
                    column(width = 2,
                           selectizeInput(inputId = paste0("var", .x + 1), 
                                          label = paste(input$var[.x + 1], "var:"),
                                          choices = c("this", "that"),
                                          multiple = FALSE))))
  })
}

shinyApp(ui, server)

Answer 1

You can detect the odd variable using is.na(input$var[.x + 1]) then span it on 4 columns as in :

row_idx %>%
  map( ~ {
    if (!is.na(input$var[.x + 1]))
      fluidRow(column(
        width = 2,
        selectizeInput(
          inputId = paste0("var", .x),
          label = paste(input$var[.x], "var:"),
          choices = c("this", "that"),
          multiple = FALSE
        )
      ),
      column(
        width = 2,
        selectizeInput(
          inputId = paste0("var", .x + 1),
          label = paste(input$var[.x + 1], "var:"),
          choices = c("this", "that"),
          multiple = FALSE
        )
      ))
    else
      fluidRow(column(
        width = 4,
        selectizeInput(
          inputId = paste0("var", .x),
          label = paste(input$var[.x], "var:"),
          choices = c("this", "that"),
          multiple = FALSE
        )
      ))
  })

Answer 2

What about something like the following?

library(tidyverse)
library(shiny)

column2 = function(x, input) {
  column(
    width = 2,
    selectizeInput(
      inputId = paste0("var", x), 
      label = paste(input$var[x], "var:"),
      choices = c("this", "that"),
      multiple = FALSE
    )
  )
}

ui <- fluidPage(
  selectInput(inputId = "var",
              label = "vars:",
              choices = colnames(mtcars),
              multiple = TRUE),
  uiOutput("dynUI")
)

server <- function(input, output, session) {
  
  output$dynUI <- renderUI({
    row_idx <- length(input$var) %>% seq_len
    row_idx <- split(row_idx, (seq(row_idx) - 1) %/% 2)
    map(row_idx, function(x, input) fluidRow(map(x, column2, input = input)), input = input)
  })
}

shinyApp(ui, server)

EDIT: